The present invention relates to a conductive, tenacious and protective coating for a metallic substrate, such as steel, aluminum, iron, platinum, silver, nickel, gold, cobalt, copper or copper alloys and zinc. More particularly, the present invention relates to a method of grafting a conductive and protective coating containing nickel on a metallic substrate, such as steel and aluminum, to form a coating that not only protects the substrate but also provides minimal electrical resistance. The present invention also relates to a composition for graft polymerizing a conductive, tenacious, and protective coating on a metallic substrate and to a metallic substrate coated with a conductive, tenacious, and protective coating.
In the computer and electronics industry, metallic portions of an apparatus or equipment require protection against corrosion. Any corrosion protection, however, should not diminish the electrical conductivity of the metal. Such protection should also be sufficiently durable so that it is not readily removed from the metal as the metal undergoes abrasion and exposure to the environment during routine use.
Providing Electro-Magnetic Interference (EMI) shielding for computers and other electronic devices in order to comply with both regulatory and functional requirements often requires the designers to provide a highly conductive enclosure that completely surrounds the device to the maximum extent possible and with the lowest surface resistivity (Rs) possible. This concept is often referred to as creating a `Faraday Cage` around the product and is highly effective in containing electromagnetic fields and also in preventing their entry.
In addition to providing a low resistance for electro-magnetic interference (EMI) currents, the enclosures must also meet electro-static discharge (ESD) requirements where similarly low surface resistivities must be achieved. Since these enclosures may be exposed to environmental conditions such as a relative humidity of 75-80%, a temperature exceeding 100.degree. C., chlorine, hydrogen sulfide, nitrous oxide, and oxidation, the metal portions of the enclosures need to be protected so that the metal does not corrode.
In order to accomplish these objectives, the mating surfaces of the metal parts used to create the enclosure must be joined in a manner to minimize electrical resistance across the joints. For EMI purposes, conductive gaskets, spring fingers, and other appliances are commonly used in these areas to provide maximum contact area between the mating pieces with the minimum use of fasteners. It is in these areas, particularly, that the metal surfaces be corrosion resistant, tenacious, and have the lowest possible surface resistivity. It is also desirable to minimize galvanic corrosion that can occur when dissimilar metals come into contact. It is generally accepted that surface resistivities of 0.1 ohm/square or less are necessary to maintain the shielding effectiveness required for these applications.
It is, of course, equally critical to have low surface resistivities where metal to metal seams and overlaps occur without gasketing.
Presently, many different types of surface treatments are used in an effort to achieve the necessary requirements mentioned above. These include conductive paints, yellow and clear chromate coatings and precious and non-precious metal platings. Each of these methods have cost and/or performance deficiencies. Surface resistivity, stability and adhesion are all problems with the paints. The yellow and clear zinc chromates are easily displaced and are basically non-conductive; thus, they rely on being displaced to provide conduction to the metal substrate. This, in turn, creates an unprotected void on the metal surface which can subsequently corrode. The metal platings are generally expensive and, in some cases, are not tenacious enough for the abrasion environment encountered in these products.
Zinc chromates are coatings that are used for protecting metal surfaces and are not inherently conductive. Placing chromate coated sub-assemblies together to build an enclosure does not guarantee electrical conductivity between subassemblies due to the non-conductive property of the zinc chromate. The uncertainty of electrical conductivity between the outer subassemblies in an enclosure makes the enclosure less than ideal for EMI shielding and ESD protection.
For example, spring contacts, sometimes called finger-stock, attached to an enclosure, bulkhead or subassembly, are intended to make electrical contact with mating parts of the enclosure or bulkhead or subassembly in order to create an electrically continuous shield. Zinc chromate coatings are easily displaced and spread to allow the spring contact to make contact with the metal substrate, thus making electrical contact with the metal substrate. As the zinc chromate coating is displaced to allow electrical contact, the metal substrate becomes exposed to the environment and subsequently corrodes.
In addition to high cost, the metal platings are also subject to wear, vibration, and handling problems. As these coatings are applied at minimal thicknesses to minimize cost and are physically rather than chemically bonded to the substrate materials, these are susceptible to removal by `normal` abrasion when used with working gaskets and spring contacts. These platings are also susceptible to galvanic corrosion when contacting dissimilar metal surfaces. There is also a phenomena known as `fretting corrosion` which occurs with minimal mating pressures and is associated with metal migration/loss across the boundary area. In these cases, base metal corrosion occurs and environmental protection is lost. Also, in some instances, aesthestics are impacted due to surface contamination and discoloration. Therefore the tenacity of these coatings is not sufficient for electro-magnetic compatibility (EMC) requirements of enclosures.
These bulkheads may form a physical barrier that prevents operator access. Spring contacts attached to one bulkhead make contact with the adjacent bulkhead. When grounded, this results in an effective electrical shield. If the bulkheads are not coated to prevent corrosion, the reliability of these contacts is dramatically reduced and the customer's satisfaction with the product is adversely affected. However, as noted, any corrosion protection should not diminish the conductivity of the bulkheads.
Even when the metallic parts are coated with expensive, conductive organic paints, the coatings tend to be permeable to various corrosive gases so that the requisite degree of corrosion protection is not obtained. Because these paints typically adhere to the substrate through only physical bonds, they can be readily dislodged from the substrate over a short period of time as moisture, oxygen, chlorine, hydrogen sulfide and other corrosive gases permeate beneath the coated film.
Thus, there is a need for a coating that not only protects the metal from corrosion but also is conductive and durable. For some computer and electronic applications, a surface resistivity of less than 0.1 ohm per square is preferred on the coated metal. Of course, it is also preferred that the selected protective, tenacious and conductive coating also be relatively inexpensive and easy to apply.